This invention relates generally to improvements in gas turbine engine systems, particularly of the type including a gas turbine engine for driving an electrical generator to provide electrical power. More specifically, this invention relates to an inertial speed control device for use with a multiple shaft gas turbine engine to maintain the output speed thereof substantially constant in response to a transient load condition.
Gas turbine engines have been widely used for driving many different types of rotary equipment, such as an electrical generator to provide a source of electrical power. Typically, the gas turbine engine comprises a compressor and a turbine mounted on a common rotatable shaft in close association with a combustor. The compressor functions on rotation to draw in and compress a charge of air which is supplied to the combustor for admixture with fuel therein. The combined air-fuel mixture is combusted within the combustor to provide a high energy gas outflow for rotatably driving the turbine, thereby rotatably driving the common shaft and the compressor. In accordance with some engine designs, referred to as "single shaft" engines, the common shaft comprises the engine output shaft and is adapted for direct connection to an engine load, such as an electrical generator or the like. In other engine designs characterized generally as "multiple shaft" engines, the high energy gas outflow is further coupled to one or more so-called free power turbines carried by an independently rotatable output shaft adapted for connection to the engine load.
In some gas turbine engine systems, the single shaft engine design may be preferred. More particularly, when operated at or near rated engine speed and load conditions, single shaft gas turbine engines exhibit relatively high engine efficiency and fuel economy. Moreover, the combined rotational inertia of the compressor and turbine on the single common shaft tends to resist variations in engine output speed in response to relatively small changes in engine load. However, the single shaft engine is not well suited for some system applications requiring a substantially constant engine output speed irrespective of changing engine load conditions, such as an electrical power generation system requiring constant output speed to generate electrical power at a prescribed frequency. In such systems, the requisite constant output speed correspondingly requires a relatively high air mass flow through the engine, wherein this high mass flow results in relatively poor efficiency and fuel economy when the engine is operated at power settings significantly below its design rating.
Accordingly, in many turbine engine system applications, the so-called multiple shaft gas turbine engine is used, since the free power turbine or turbines thereof may be rotated at an output speed independent from the rotational speed of the engine compressor. Accordingly, in an electrical power generation system requiring a relatively high and relatively constant output speed throughout a range of engine loads, the free power turbine rotational speed can be maintained constant while the rotational speed of the commonly shafted compressor and turbine can be varied appropriately by adjusting combustor fuel supply to improve overall engine efficiency and fuel economy.
However, the free power turbine of a multiple shaft gas turbine engine is subject to short-term speed variations caused by relatively sudden changes in engine load. For example, a sudden step increase in engine load can cause the power turbine to slow briefly until the combustor fuel supply can be appropriately adjusted to accommodate the change in engine load. Similarly, and perhaps more importantly, a sudden decrease in engine load can cause short-term overspeeding of the free power turbine during fuel supply adjustment to accommodate the changed load. Such overspeeding of the free power turbine is undesirably accompanied by a substantially increased risk of mechanical component failure.
There exists, therefore, a significant need for a speed control device for a multiple shaft gas turbine engine for maintaining the rotational speed of the free power turbine substantially constant during a transient load condition. The present invention fulfills this need and provides further related advantages.